1.1 Field of the Invention
The invention relates to thin films comprising mesoporous transition metal oxide materials. The invention further relates to a method of producing mesoporous transition metal oxide thin films via pulsed laser ablation of appropriate mesoporous molecular sieve targets. The invention also relates to the use of mesoporous transition metal oxide thin films produced by the method of the invention to manufacture chemical sensors and electrochromic devices.
1.2 Description of Related Art
Two classes of materials with wide ranging uses as heterogeneous catalysts, adsorption media, and as components of chemical sensors and electrochromic devices, are microporous (pore diameter&lt;20 .ANG.) and mesoporous (pore diameter 20 .ANG.-500 .ANG.) inorganic solids. The utility of these materials is a consequence of their chemical structures, which allow guest molecules access to large internal void surfaces and cavities, thereby enhancing the catalytic activity and adsorptive capacity of these materials.
Typical of the microporous materials are the aluminosilicate molecular sieves known as zeolites. In zeolites, the micropores form regular arrays of uniformly-sized channels. Zeolites can act as hosts to ionic and neutral molecular guest species. However, the usefulness of sensors and devices fabricated from zeolites and other microporous materials is generally limited to those applications where the guest or analyte molecules have sufficiently small kinetic diameters to pass through the narrow microporous void openings.
Mesoporous materials offer the advantage of larger pore sizes, making them compatible with applications such as separation or sensing of relatively large organic molecules. Typical of the mesoporous materials are amorphous or polycrystalline solids such as pillared clays and silicates. Unfortunately, the pores in these materials are often irregularly spaced and broadly distributed in size, making them ill-suited for chemical separations, sensing and other device-oriented applications.
Considerable synthetic effort has therefore been devoted to developing molecular sieve frameworks with pore diameters within the mesoporous range, and a series of mesoporous molecular sieves having a hexagonal array of uniform mesopores has recently been developed (Beck et al., 1992). These materials, designated MCM-41, are of great interest because their large and uniform pore sizes allow otherwise sterically hindered molecules facile diffusion to internal active sites. However, the MCM-41 series of molecular sieves are silicate and aluminosilicate materials, and their aluminum and silicon centers do not have variable oxidation states, thus precluding the use of these materials in display applications and related electrochromic devices.
A method of precisely controlling deposition of a well-adhered mesoporous transition metal oxide thin film having redox active metal centers would be beneficial in extending the range of thin film materials available for use in applications such as electrochromic devices, chemical separations, and chemical sensing.